The present invention relates generally to satellites, and more particularly, to methods that use satellite state vector prediction to automatically inhibit satellite sensor scans and/or provide sensor switching.
The assignee of the present invention has developed a satellite that is to be deployed in a predetermined orbit around the Earth that requires yaw steering in certain circumstances. The orbit is highly inclined and elliptical. A plurality of Earth sensors, one or more sun sensors and a plurality of gyro sensors are employed to control the attitude of the satellite. Certain sensors, in this case the Earth sensor, are sensitive to intrusion by bright bodies such as the sun or the moon in their fields of view.
Such intrusions can lead to loss of attitude control and special precautions must be taken when such an event is predicted to occur. Specifically, for the Earth sensor, the scan experiencing the intrusion must be inhibited and the Earth chord it measures must be captured during the inhibit period in order for the remaining scan to be used to maintain attitude.
In addition, in the elliptical orbit where the Earth apparent radius varies, the captured Earth standard chord must be biased as the Earth radius changes for attitude control. During orbit normal operations of the satellite in this particular orbit, sun intrusions in the Earth sensor occur daily (for low sun angles). In addition to sun intrusions, moon intrusions may be encountered when in orbit normal operations.
When the satellite is operated in yaw steering mode, for low to moderate cases of sun angles, once daily, near satellite local midnight, the sun will follow a trajectory that is centered between north and south scan lines of the Earth sensor. A typical moon intrusion in yaw steering transverses through both Earth sensor scan lines in a very short time period (see FIG. 2). Moon intrusion can be expected to occur every day for four or five days in a row repeating approximately every two weeks.
Thus, the orbit and yaw steering of this satellite produce problems because of the interference of the sun and moon with the Earth sensor scans. Lunar intrusion because of yaw steering can come from any direction several times a week.
Due to the frequency of these intrusion events and the commanding required to perform inhibits manually, it would be desirable to have a method that automatically inhibits Earth sensor scans to eliminate sensor intrusions by the sun, moon, or other celestial bodies. Therefore, it is an objective of the present invention to provide for improved methods that use satellite state vector prediction to automatically inhibit satellite Earth sensor scans and/or provide sensor switching.